For many years, the red signal lamps commonly seen at railroad crossings have included a roundel type lens which disperses light from a bulb behind the lens in essentially equal proportion to areas in front of, to both sides of, above and below the lamp. Provision is frequently made for passing a certain amount of light straight through the lens without dispersion, to provide adequate brightness at long distances in front of the lamp. The familiar signal lamp includes a long hood or visor which keeps snow and ice from obscuring the lens and a circular background ring surrounding the roundel which provides contrast with ambient light sources. A variation of this familiar lamp, disclosed in U.S. Pat. No. 3,597,606, includes a lens set at an angle at one end of an elongated housing cover, so that the need for a visor is substantially eliminated.
The regulations of the Association of American Railroads require that each crossing signal be provided with a standby battery for use during emergencies so that signal operation will be ensured for a specified length of time at specified light intensities in certain areas before the signal lamp. To satisfy these requirements, rather large capacity batteries are required, at considerable expense to the railroad companies. One reason that such large batteries must be used is a function of the design of the lenses commonly used in prior art signal lamps. As previously mentioned, the prior art lenses disperse light to the entire area in front of the lamp. Thus, light is directed at the fields or buildings adjacent the roadway which crosses the railroad tracks, at the ground in front of the signal, as well as into the eyes of vehicle operators approaching the crossing on the highway. Clearly, this results in a waste of much of the power of the emergency battery to illuminate areas other than the adjacent highway. If means were provided for directing the light preferentially toward the roadway while still ensuring adequate brilliance at other angles, then the battery requirements would be reduced with attendant savings. A lower power battery could be used to produce the required light intensity in the highway area where the signal must be seen at all events and reduced intensity in other areas. Conversely, present batteries could continue to be used with increased light intensity in the highway area.